In the technical field of an automatic character printer, an electronic guiding apparatus, an information retrieval apparatus or the like, an economical and small display apparatus capable of displaying the same amount of information as recorded on one recording sheet, or a display apparatus using a recording paper which can be used repeatedly, has widely been desired.
One example of the conventional display apparatus of this type is proposed in U.S. Pat. No. 3,219,993. According to this prior art technique, a thermally reversible medium is used as a display medium, and by switching the direction of an electric current passing through a Peltier junction mounted on the display medium, the display medium is heated to effect recording or is reversibly cooled to erase the displayed record. In this prior art technique, however, it is very difficult to accumulate such Peltier junctions playing an important role for the display at such a high density as 64 picture elements per square millimeter, and such high-density accumulation of Peltier junctions results in increase of the manufacturing cost of the display apparatus. Furthermore, in this conventional apparatus, each Peltier junction should electrically be connected through a metal having a good thermal conductivity, which causes problem of conduction of heat among adjacent Peltier junctions and also causes reduction of the recording quality.
Another conventional apparatus of this type is disclosed in Japanese Patent Publication Ser. No. 52-49948. In this conventional apparatus, a thermo-reversible display medium is used as in case of the above-mentioned conventional technique, and recording is effected by instantaneously heating thermal elements accumulated on the display medium and then, the display medium is rotated and transferred from the display surface to another position to effect cooling for erasing the record. In this conventional apparatus, the color change for recording is a change of yellow to yellowish orange or red to reddish violet, and therefore, the contrast between the two colors is not sufficient. Moreover, these colors are problematical from the standpoint of the human engineering, but this problem is not taken into consideration in the prior art technique at all. Furthermore, the hysteresis characteristic of the display medium involves a problem of the thermal inertia, but the problem is not at all taken into account. The problem of thermal inertia referred to herein is that when the frequency of the repeated use of the display medium is high, for example, about one minute, the color of the recorded area cannot be restored to the original color only by cooling the display medium and a residual image is left at the subsequent display operation. This problem also is involved in the first-mentioned prior art.
It is therefore a primary object of the present invention to provide a small and economical display apparatus in which high-density display can be accomplished on a thermal display medium which has a hysteresis effect between the temperature and color.
Another object of the present invention is to provide a display apparatus in which the quality of a picture pattern on the display surface can be improved by using novel lighting and color filter means.
Still another object of the present invention is to provide a display apparatus in which the displayed record can be erased promptly and uniformly when the display medium is repeatedly used.